Sand

Potential problem:       Tendon/ligament injury.

How it can happen:      Believe it or not, trail horses tend
to suffer more leg injuries from soft ground than hard
ground.

In sandy or boggy footing, the extra stress put onto your
horse’s legs can result in superficial flexor tendon and
suspensory ligament injuries. Superficial flexor tendons
run down the back of the foreleg, between the knee and
foot. In the hind leg, they run between the hock and the
foot; they cause your horse’s joints to flex when
stimulated. Suspensory ligaments run behind the cannon
bone over the fetlock joint to the pastern bones; they
support the fetlock joint, which is your horse’s ankle.      
Below is an excerpt from an article posted on myhorse.com .  It would apply to our arena after a rain.  For the entire article,
Click Here.  It's four pages long so make sure you see them all.  Very informative!
The biggest culprit is fatigue. Soft footing can tire your horse sooner than you’re used to and to a much greater degree than
you might imagine, simply from the extra work he must expend to get through it. His joints must flex more and his legs must
move more distance at every stride. More flexing means more pull on tendons and ligaments, because the foot didn’t stop
soon enough; it keeps slipping a little on every stride. This wasted motion with every step leads to fatigue — and fatigue
leads to tendon and ligament injuries. Note that such injuries don’t always occur near the end of a long ride; your horse can
go lame at any point if he’s not in condition.
Leg Injuries & Causes
Rocking Z Stables